The need to operate sensors in harsh environments is common. Examples include sensors, switches, and the like, which are designed to operate in abrasive environments, in corrosive chemicals, and the like. Oftentimes the long term sensor operation is hindered by the continued contact the environment. The problem is particularly severe in the field of electro acoustic sensors, also known as Acoustic Wave Devices. Thus the present specifications will relate to the application of a coating to mitigate the problems associated with such operations, primarily by using the AWD sensor. The skilled in the art will recognize the applicability of the coating to other devices.
Piezoelectric sensors are well known. They are used for sensing material properties such as viscosity and density, for detecting the presence of certain materials in an environment, for measuring purity of fluid substance, and the like. Structures known for acoustic sensing range from the simple crystal resonator, crystal filters, acoustic plate mode devices, Lamb wave devices, and others. Briefly, these devices comprise a substrate of piezoelectric material such as quartz, langasite or lithium niobate, or thin films of piezoelectric material, such as aluminum nitride, zinc oxide, or cadmium sulfide, on a non-piezoelectric substrate. The substrate has at least one active piezoelectric surface area, which is commonly highly polished. Formed on the surface are input and output transducers for the purpose of converting input electrical energy to acoustic energy within the substrate and reconverting the acoustic energy to an electric output signal. These transducers may consist of parallel plate (bulk wave) or periodic interdigitated (surface-generated wave) transducers. Each sensor has at least one sensing area which is exposed to the environment being measured. The interaction between the surface and the environment causes measurable changes in the electrical characteristics of the sensor. The sensors may be used for sensing density, viscosity, and other such physical parameters.
Piezoelectric devices are generally manufactured from hard, crystalline materials. However, even those surfaces change when exposed to certain chemicals or abrasives. Piezoelectric based sensors are very susceptible to changes in the sensing area. Thus their use was so far limited to environments that will not damage such surfaces. Damage may be chemical such as etching, or mechanical such as abrasion. Therefore the usability of such sensors in environments like oil wells measuring of characteristics of drilling mud or oil, sensing characteristics of ink, melted polymers, and similar abrasive materials was heretofore limited as the sensors will suffer from high variability over time. Other environments which can harm such sensors are chemically reactive materials such as strong acids and bases used in polymer processing, pulp and paper processing, and other industrial and chemical processes. Furthermore, the need for a conductive electrode or shield layer in the most desirable sensor topologies introduces a further susceptibility to chemical attack and/or abrasion as virtually all metals have one or more chemical susceptibility and or are soft, abrasion prone materials. A further common requirement to liquid phase sensing is that the sensing surface be sufficiently smooth. Many liquid phase AWD sensors require nanometer or lower average roughness.
The electrodes that are deployed in the most desirable sensor topologies are commonly made of gold (Au) deposited by any convenient method. In many cases the electrode have to be electrically insulated from the environment.
Insulating surfaces, devices, electrodes, and the like, from an hostile environment is commonly done by coatings the likes of plastic, glass or similar materials. However such coatings often interfere with the operation of the sensor. For example in an AWD type sensor, the coating must have acoustic qualities that will not significantly impede the sensor operation, as well as the desired hardness, toughness, electrical characteristics, and the like. Plastics and glass incur excessive damping and are not always chemically resistant, nor are they sufficiently hard.
Diamond Like Carbon, or DLC hereinafter, is well known coating material. DLC enjoys high hardness and therefore high resistance to abrasion, it may be smoothly applied, and generally provides an excellent coating layer whose thickness may be tailored to need. DLC is deposited using common methods such as vaporizing, ion implanting, and the like.
Certain materials do not adhere well to each other. DLC suffers from poor adhesion to materials like gold, platinum, silver, most oxides, and many other materials, especially piezoelectric materials commonly used in AWD sensors and the like. Therefore, while the DLC clearly provides the required abrasion resistance, providing adequate adhesion between a DLC layer and a piezoelectric material or an electrode deposited on such piezoelectric material presents a problem. In other cases, different coating materials exhibit undesirably large ion migration problems which adversely effect electrical or acoustic characteristics of the desired coating.
Implantable medical devices have been demonstrated with relatively thick (in the order of several to several hundred microns) coatings of DLC, on thin (a few nanometers) adhesion layers, as shown in “EXPERIMENTAL STUDIES ON DIAMOND-LIKE CARBON AND NOVEL DIAMOND-LIKE CARBON-POLYMER-HYBRID COATINGS” Mirjami Kiuru, PhD Dissertation, University of Helsinki (2004). Typical substrates for such coatings are refractory metal parts such as titanium hips. When these coatings are applied to semiconductor devices the films are extremely unreliable. These coating films often suffer from delaminate and in some cases cause breakage of the silicon substrates.
Therefore, there exists a continuing and yet unresolved need for a sensor capable of continued operation in broad range of chemical, thermal and mechanical harsh environments.
It is therefore an object of the present invention to provide a coating that will provide the above mentioned characteristics when applied to a sensing area of a sensor. Since no single material offers all of the ideal properties, many applications will ultimately require a series of layers, relaxing the requirements of the individual layers but further requiring compatibility amongst the layers. Preferably the coating should be sufficiently smooth to address fluid-phase sensor applications.
Many such acoustic sensors utilize an electrode on the sensing surface, and the coating provided by the present invention is particularly beneficial to such sensors. The skilled in the art will recognize that the term electrodes may relate to ground electrodes, transducers (especially in the case of actively driving or driven electrodes), other structures that cause perturbation or reflection in the piezoelectric crystal, or other conductors that either carry electrical energy or deliver it to a predetermined point.
An additional objective of the present invention is to provide the sensor with a smooth (preferably nanometer scale) sensing surface with excellent abrasion resistance, excellent chemical resistance and the ability to withstand temperature extremes from −50° C. to +350° C. Suitable coatings include alloys of silicon-aluminum oxynitride (SiAlON) including the extremes of silicon nitride, aluminum oxide and the like, amorphous boron nitride, amorphous and nanocrystalline carbon, boron carbide (including the like of boron doped diamond and boron doped DLC), and β-C3N4. All of these materials offer abrasion resistance and thermal stability with varying degrees of chemical resistance.
Of these coatings, so-called diamond-like carbon (DLC) is found to offer the best properties of surface smoothness, chemical resistance and abrasion resistance. While there is considerable debate as to an exact definition of diamond-like, for the purposes of the present invention it shall be taken to imply all films with a molar percentage of greater than 75% carbon and having mixed chemical bonding state of graphitic (sp2) and diamond (sp3). It should be noted that the term also extend to various modification of diamond-like carbon, such as boron-doped diamond (BDD), carbon-rich refractory metal carbides, and the like.
Diamond-like carbon is well-adhered to metals that form carbides, such as tungsten, molybdenum, tantalum, niobium, vanadium, hafnium, zirconium, titanium, and chromium in increasing order of typical adhesive strength. These metals are traditionally used in so-called adhesion layers. The adhesive strength of these metals is in direct proportion to their ability to inter-diffuse and alloy with adjacent materials.
Inter-diffusion of the adhesion layer into the DLC film is undesirable since it leads to unstable film properties and poor aging characteristics. It is especially desired that the underlying metal have low mobility in the carbon and that metal will have low mobility into the carbon, a condition that may become critical at high temperatures. Therefore by way of example, while titanium and zirconium offer excellent adhesion, they are excessively mobile in carbon and vice versa at high temperatures. On the other hand tungsten is an excellent barrier metal (has low mobility and prevents other atoms from diffusing into or through it); however tungsten has the poorest adhesion. Niobium and tantalum are the preferred metals for a barrier/adhesion layer under diamond-like carbon.
Niobium and tantalum offer good chemical resistance and excellent adhesion of the outermost DLC layer. Both materials are sufficiently conductive for shielding but are inadequate for many electrode requirements. In these cases an innermost layer of a thermally stable material with chemical resistance and high conductivity is required. Although aluminum is frequently employed as an electrode material it is chemically active and highly mobile at elevated temperatures. The preferred metals are gold and platinum, although silver and palladium are also acceptable for some preferred embodiments. Platinum has the most favorable properties while gold is more commonly employed. Ruthenium, Rhodium, Rhenium, Osmium and Iridium may also prove desirable in specific applications.
Applying the coating to a piezoelectric material presents yet another problem stemming from the extremely high film stresses associated with DLC and the significant mismatch of thermal expansion between DLC and such metal or piezoelectric materials as are commonly employed. The preferred embodiment of the present invention therefore utilizes application of a relatively thin DLC coating (of less than 1 μm) and a thicker adhesion/barrier metal system than is traditionally employed (of about 200 nm).
The adhesion of tantalum to gold or platinum and of the electrode to the piezoelectric substrate can be improved through a thin adhesion layer of chromium or titanium. The most preferred embodiment would consist of a thin (Circa 10 nm) titanium, zirconium or chromium layer, a conductive (50-200 nm) gold or platinum electrode layer, another thin (Circa 10 nm) titanium, zirconium or chromium layer, a barrier layer (25-300 nm) of niobium or tantalum, and a surface of DLC (10-500 nm).
The exact composition of the DLC is a matter of choice. Prior art in thin film development suggest numerous dopants or surface treatments ranging from several parts per million to several percent of nitrogen or various metals. Details of thin film may be found in “Synthesis and Evaluation of TaC:C Low-Friction Coatings”, Daniel Nilsson, Ph.D. Dissertation, ACTA Universitatis Upsaliensis (2004). Fluorine dopants have been considered from trace levels through 67 atomic percent (perfluoroalkanes). The abrasion and chemical resistance of such films appear best when the atomic percent carbon is maximized.
Films with only 33% carbon include the limit of hydrocarbons and fluorocarbons (e.g. Teflon®) and have no abrasion resistance. Films in the range of 33% to 66% carbon are characteristic of carbide alloys. These films are extremely hard but are not as chemically resistant or as thermally stable as DLC. In many cases the surfaces are not as smooth as true DLC. Thus only films containing approximately 67% or higher percentage carbon in their bulk are specifically considered as “DLC” herein. Our experience with tantalum-doped and fluorine-doped films indicates that the film should preferably have in excess of 90% carbon and the most preferred embodiment has in excess of 97% carbon.
While chemical resistance and passivation generally favor an insulating layer as is obtained by pure carbon DLC, certain sensor applications using carbon electrodes in electrochemical sensors, notably electrochemical-AWD hybrids, require a conducting DLC layer. Boron doped (˜1019 cm−3, consistent with silicon doping levels, up to 1% B; 99% C) diamond is a suitable material for use in electrochemical and/or acoustic sensors, and is considered as a type of DLC.
A further object of the invention includes the ability to attach chemically selective probes to the DLC surface.
Thus in one aspect of the present invention there is provided a coated Acoustic Wave Device (AWD) based sensor, the sensor having at least one piezoelectric plate having two opposing faces, one of said faces having a sensing area, the sensor having a coating applied to the sensing area, the coating characterized by: a first electrode disposed on the sensing area; a barrier layer disposed over the first electrode, the barrier layer comprising at least one metal selected from the group consisting of tantalum, niobium, vanadium, molybdenum, or a combination thereof; and an abrasive resistant layer comprising Diamond-Like Carbon (DLC) disposed over the barrier layer.
Optionally the coating further comprises an adhesion layer disposed between the barrier layer and the electrode. Further optionally another adhesion layer may be disposed between the electrode and the sensing face. The adhesion layers may be titanium, zirconium, chromium, vanadium, niobium, tantalum, molybdenum, or a combination thereof.
Preferably the first electrode comprises a metal selected from a group consisting of platinum, palladium, gold, silver, copper, aluminum, osmium, iridium, or a combination thereof. Optionally the DLC may be boron doped diamond. Further optionally, chemically selective probes are coupled to the DLC.
In a preferred embodiment of the present invention there is provided a coated acoustic wave device sensor as described above, having a second electrode disposed on the face opposite the face comprising the sensing area, forming a parallel plate resonator with said first and second electrodes. Optionally, a third electrode is disposed on the face opposite the face comprising the sensing area, wherein the first and second electrodes form an input parallel resonator, acting as an input transducer, the first and third electrode also form an output parallel resonator, acting as an output transducer; wherein the input and output resonators being sufficiently close to couple acoustic energy from the input resonator into the output resonator, for forming a multi-pole coupled resonator filter.
In another aspect of the present invention there is provided a method of passivating an electrode, the method characterized by the steps of:                depositing a barrier layer onto said electrode, the barrier layer comprising at least one metal selected from the group consisting of tantalum, niobium, vanadium, molybdenum, or a combination thereof; and,        depositing an abrasive resistant layer comprising Diamond-Like Carbon (DLC) onto said barrier layer.        
Optionally the method further comprises the step of depositing an adhesion layer between the electrode and the barrier layer. Most preferably the electrode is coupled to an acoustic wave device based sensor. Optionally, the method further comprise the step of coupling chemically selective probes to the DLC layer.